(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a copier and a printer, and particularly relates to an improvement on a transfer technique used by an image forming apparatus in which toner images are transferred at a plurality of positions, such as a tandem-type image forming apparatus.
(2) Description of the Related Art
When a toner image transfer is performed using the electrostatic transfer method, a toner image formed on a photosensitive drum is transferred onto a recording sheet as one example of conventional ways. Specifically, a transfer roller, which is set facing the photosensitive drum, applies an electric field so that the polarity of the transfer roller side is opposite to the polarity of toner. By means of an action of the electric field, the toner on the surface of the photosensitive drum is attracted to a surface of a recording sheet which passes between the photosensitive drum and the transfer roller.
Here, the resistance of the transfer roller varies with surrounding conditions, such as temperature and humidity. As such, the transfer current varies even though the voltage applied to the transfer roller is controlled to be constant, thereby making the toner image transfer unstable.
Tandem-type color copiers have received much attention in recent years as image forming apparatuses which can perform color printing at high speed. However, the stated problem occurs to a tandem-type color copier having transfer devices corresponding to photosensitive drums set along a transport belt.
To address this problem, Japanese Laid-Open Patent Applications No. 2-123385 and No. 7-120117 teach the following method although each of their inventions relates to a copier having a single transfer device, and does not relate to a copier having transfer devices such as a tandem-type copier.
More specifically, before an image forming process is executed, an optimum current is applied to the transfer roller and a voltage obtained by means of the application of the optimum current is measured. Then, a voltage (i.e., an optimum voltage) based on the measurement result is applied to the transfer roller when the image forming process is executed. By this method, excellent toner image transfer can be performed regardless of the change in the surrounding conditions.
However, using the image forming apparatus such as the tandem-type color copier, transfer deterioration still occurs even when image formation is executed after the optimum voltage is obtained for each transfer roller using the stated method, since more than one transfer device (or, transfer roller) is provided.